Heating of flat glass sheets is performed to provide bending, tempering, bending and tempering, heat strengthening, and pyrolytic filming, etc. Usually the glass is heated above its strain point which is the temperature at which the glass acts as a viscous fluid rather than an elastic solid. The heated glass sheets are thus easily subjected to unintended deformation when heated to the viscous condition, and great care must be taken if the glass sheets are to have the required optical quality after subsequent cooling.
Historically, most commercial heating of glass sheets until the early 1960s was performed by the vertical process where tongs are utilized to suspend the upper edge of the glass sheet which hangs downwardly and is conveyed through a heating chamber for the heating. One problem with this vertical process is that the entire weight of the glass sheet is supported by the tongs and, upon being heated sufficiently hot to become viscous, the glass sheets tend to deform at the tongs which leaves "tong marks" upon subsequent cooling. Also, furnace capacity can be wasted in the vertical process since short glass sheets require the same conveyor usage as long glass sheets.
U.S Pat. Nos. 2,841,925 of McMaster and 3,402,038 of Hordis disclose glass tempering furnaces of the vertical type described above. Each of these vertical furnaces is disclosed as including fans for circulating gas to provide uniformity in the heating. However, in commercial units manufactured in accordance with these patents, the gas pressures utilized have been relatively low, only on the order of about one-half inch water column at the output of the fans. As such, the amount of forced convection is not particularly great and the dominant mode of heating is by radiation from the furnace walls and other components of the furnace such as the fan blowers and associated baffle plates, etc.
In an attempt to overcome problems associated with vertical type furnaces for heating glass sheets, gas hearth furnaces were developed during the early 1960s. This gas hearth type of furnace includes a generally horizontal but slightly inclined hearth through which gas is supplied to provide a thin layer of gas on which the glass sheets are supported during heating. A pressurized plenum below the hearth supplies the gas through openings in the hearth to support the glass sheets for conveyance. Recirculation of the gas between the furnace and the plenum provides the glass sheet support without heat loss that would result if the gas were allowed to escape to the atmosphere. At the lower edge of the tilted hearth, a movable frame is provided to provide movement of the glass sheets along the hearth on the thin layer of gas provided by the pressurized plenum. Heating of the glass sheet is thus performed by gas supplied by the hearth which constitutes a part of the conveyor of this type of furnace. Also, substantial radiant heat transfer takes place between the hearth and the lower surfaces of the glass sheets conveyed on the hearth. Substantial radiation also takes place between the furnace walls and the upper surfaces of the conveyed glass sheets. In addition, as disclosed by the U.S. Pat. No. 4,059,426 of Starr, gas heaters have previously been utilized to provide forced convection heating of the upper surfaces of the glass sheets.
Roller conveyor furnaces for heating glass sheets did not receive any widespread commercial acceptance until introduction of the frictionally driven roller conveyor furnace for heating glass sheets as disclosed by the U.S. Pat. No. 3,806,312 of McMaster and Nitschke. Thereafter, further commercial acceptance of roller conveyor furnaces for heating glass sheets was achieved upon introduction of the furnaces disclosed by the U.S. Pat. Nos. 3,934,970 and 3,947,242 of McMaster and Nitschke. Subsequently, the oscillating type of roller conveyor furnace for heating glass sheets, as disclosed by the U.S. Pat. No. 3,994,711 of McMaster, received further commercial acceptance. All of these roller conveyor furnaces utilized electric resistance heaters for providing radiant heat transfer as the dominant mode of heating the glass sheets.
U.S. Pat. Nos. 4,505,671 and 4,529,580 of McMaster disclose glass sheet heating by the use of forced convection. In the U.S. Pat. No. 4,505,671 McMaster patent, the forced convection heating is disclosed as providing glass temperature control that maintains planarity of glass sheets during tempering. In the U.S. Pat. No. 4,529,580 McMaster patent, the forced convection heating is disclosed as providing the primary source for heating the furnace in which glass sheets are heated prior to quenching that tempers the glass sheets.
In glass sheet radiant heating, radiant energy emitted from electric resistive elements operating in the 650 to 750 degree Centigrade temperature range is primarily absorbed by a thin layer of the glass surfaces. Therefore, the edges which are heated by three surfaces will become hotter than the central areas which are heated by only two surfaces. During subsequent cooling, the hotter edges will cool faster than the center since the cooling rate is proportional to the temperature differential between the glass and the ambient air or the quenching air if the glass is to be tempered. The faster cooling tensions the glass edges relative to the center such that tension cracks tend to result. When glass is being quenched for tempering, almost all quench breakage starts at the glass edges. Reducing the probability of edge breakage by forced convection heating, which does not overheat the edges, allows tempering to be accomplished at a lower overall temperature. Lowering the temperature by only about 10 degrees Centigrade during tempering doubles the stiffness of the glass and thereby reduces distortion of the tempered glass.
Also, roller conveyor heating of glass sheets necessarily involves a certain amount of increased lower surface heating due to radiation and conduction from the rolls. Upon subsequent quenching to provide tempering of the heated glass sheets, the hotter bottom surface will shrink more than the top surface if the heat transfer rates on the two surfaces are identical. This hotter bottom surface causes the glass to arch upwardly in the center if equal pressure of quenching gas is supplied from both above and below. As such, increased pressure must be utilized at the bottom surface, which results in the glass sheet curling down around its edges and thereby distorting planarity. While this "edge" distortion problem is present at all edges of the glass sheet, it is a particular problem at the leading edge that initially enters the quench station before the rest of the glass sheet.